It is generally known that hard facing metals on such areas as valve seats, leading edges of turbine buckets, cavitation prone pump parts, and in design areas requiring changes of direction of particulates and fluids, frequently are exposed to high levels of differential erosion wear. A common procedure to reduce the rate of erosion on the surface of such hard facing metals is to flame spray or plasma spray the exposed metal surface with powdered metal such as 450Ni or a metal oxide powder. For example, zirconium oxide, or blends of zirconium oxide, calcium oxide, or yttrium oxide, etc., can be plasma sprayed to produce an adherent ceramic protective coating on the surface of a hard facing metal to extend its service life. Typical examples of the use of powdered of metallic oxides or metals to coat superalloys used in the turbines of jet aircraft are shown by Palmer et al, U.S. Pat. No. 4,055,705, and Weatherly et al, U.S. Pat. No. 4,095,003, wherein particulated metal is plasma sprayed onto the surface of the superalloy.
Although the use of such protective metallic or ceramic coatings on the surface of such hard facing metals has been found to satisfactorily extend the service life of metallic substrates subjected to a high degree of differential surface erosion or wear, it has been found that the maintenance of such coated metallic structures is often complicated. It is often difficult, for example, to predict when the protective coating on the treated substrate has to be renewed before undue exposure of bare metallic substrate surface occurs resulting in severe corrosion or erosion problems.
A procedure which has been suggested for indicating the rate of erosion of a protective coating on a metal substrate is by using an indicator, such as bismuth trioxide, which varies from yellow to deep purple in color in an undercoating, as shown by Fontanella, U.S. Pat. No. 3,508,890. Another technique which has been used by the paint industry to indicate thin areas of coverage on a metal substrate is the use of a small amount of a fluorescent, or phosphorescent material in the primer or ground coat. However, a fluorescent material, such as zinc sulfide would not be suitable under plasma or flame spraying conditions, requiring temperatures of about 2000.degree. C. or more.
The present invention is based on the discovery, that certain UV sensitive metal oxide phosphors, for example, an alumina-based phosphor, having the nominal formula, Ce.sub.1--X--Y La.sub.X Tb.sub.Y MgAl.sub.11 O.sub.19, where O&lt;X&lt;0.2 and 0.2&lt;Y&lt;0.4, cerium magnesium aluminate doped with +3 terbium, or yttrium oxide doped with +3 europium will fluoresce when utilized in an effective amount after being plasma sprayed or flame sprayed as an undercoat mixture onto various metal substrates. This result is quite surprising particularly in view of the fact that the phosphor in the indicating coating mixture is capable of surviving under plasma spraying conditions.